1. Field of the Invention
The invention relates to a testing method and apparatus for the testing of electrical insulation of electrical power apparatus, especially high and medium voltage cables with extruded polymeric insulation, and large electrical rotating machinery. In particular the invention relates to a method and device known as a Pulsed Resonant Power Supply which produces high voltage modulated line frequency output to the test sample with no dc content. There is no switching involved hence the test source is suitable for use in non-destructive testing, such as for measurement of partial discharges or dissipation factor, as well as for destructive or withstand testing.
2. Description of the Prior Art
Electrical testing of installed high-voltage power cables is generally performed at or prior to the time of installation and periodically after installation as a routine maintenance test. These tests are made to detect defects or deterioration in the cable that could cause problems during service and to verify proper installation.
Electrical maintenance testing of large rotating machinery is generally performed to assess the condition of the winding insulating materials and monitor trends with time.
The test methods used in both cases are either withstand tests (go/no-go testing) or tests used to measure some specific property of the insulation (nondestructive testing). In the case of withstand testing, the voltage to which the insulation is subjected is higher than the maximum working voltage and is generally specified in the pertinent industry standards. Usually, nondestructive testing is performed at voltage levels up to the maximum working voltage.
Tests in both categories can be either ac or dc although ac tests simulate operating conditions more nearly than dc tests and are also capable of revealing more information about the insulation condition. Ac tests permit the measurement of several different parameters that are sensitive to such things as moisture, dirt, insulation delamination and other forms of damage. Testing using an ac power source is therefore the logical choice for diagnostic testing; however, the size and weight of ac test equipment capable of supplying the reactive volt-amperes required to charge the capacitance of a large winding or long length of cable is prohibitive. Even with the availability of ac resonant test equipment and very low frequency (VLF) test equipment, dc equipment is still less bulky and expensive, and has become the preferred test method.
Dc testing, however, is not without its disadvantages, particularly in testing installed power cable. The following problems that are cause for concern have been identified with dc testing of extruded polymeric cables:
a) The dc test is inadequate for determining the ac withstand strength of extruded polymeric cables and their accessories. Certain defects, undetectable with dc, can cause breakdown under ac conditions. PA1 b) The dc test can cause premature service breakdown by accelerating certain deterioration mechanisms that would otherwise prove harmless under ac stress. PA1 c) External flashover on terminations during dc testing causes a travelling wave that can produce a severe over-voltage condition and subsequent insulation damage. PA1 d) The dc test will always result in a space charge remaining in the cable insulation. Upon re-energization with ac, field enhancements occur which can lead to insulation breakdown. PA1 a) The large dc content of the waveform, which could result in a high trapped space charge. PA1 b) Wave shape dependance on the length and capacitance of the cable under test. PA1 c) Difficulty in adapting such a power supply as a partial discharge test source due to the large switching transients involved.
Another major disadvantage with dc when testing any type of insulation is that the voltage distribution is different from that to which the insulation is subjected during normal operation and therefore these tests can never simulate actual operating conditions.
Several testing methods and apparatus have been devised in the relatively recent past in attempts to simulate operating conditions more closely, yet maintain the advantages of the small size and weight of dc apparatus. However, none of the new methods introduced has completely overcome all of the aforementioned disadvantages.
One of the earliest test apparatus of recent prior art was the resonant power supply. Either series or parallel utilizing a variable inductive reactor or a variable frequency voltage source. While producing pure sinusoidal ac and overcoming the high power input requirements of conventional ac equipment, the resonant power supply still exhibits considerable size and weight for convenient portability.
The sinusoidal very low frequency VLF test apparatus was introduced thereafter. This VLF device generates high voltage at a test frequency of usually 0.1 Hz. The VLF techniques produces an acceptable (generally less than 5% distortion) sinusoidal output, albeit at 0.1 Hz, along with a low power input requirement. Even so, the VLF apparatus, although smaller than an equivalent resonant power supply is still bulky and cannot be conveniently transported from site to site.
A modification to the VLF technique was later introduced in an attempt to reduce test apparatus size and weight by making the output waveshape nonsinusoidal. This was done by periodically switching positive and negative polarity dc on to the cable. During the switching process the cable was discharged through a choke before the opposite polarity dc power supply was connected. The result is an essentially square wave output by which the cable was subjected to opposite polarity dc every 5 seconds. This system still maintains some of the disadvantages of dc such as a gradual trapped space charge build up in the cable and a voltage distribution that does not simulate actual operating conditions. Furthermore the test voltage level for withstand testing, according to some industry standards, is still required to be several times greater than the maximum operating voltage, thus subjecting the cable and accessories to possible damage from travelling waves caused by external flashover.
The most recent prior art test method to be introduced is known as the oscillating wave method. The simplest and most efficient circuit for producing the oscillating wave comprises use of a dc power supply which charges the cable under test, together with a switch, which when closed, discharges the cable through an inductor.
There are other circuits which have been proposed in an attempt to avoid potential problems arising from dc polarization of the cable during the initial charging time. However, complications in voltage control, equipment complexity, and poor circuit efficiency have made the use of these circuits impractical. The oscillating wave variable parameters are frequency and damping rate of the oscillations, and charging time (polarization time) of the cable under test. These parameters are usually not under the control of the operator and thus characterization of the output waveshape is extremely complicated. Thus, although the size and weight of apparatus embodying such other circuits might be acceptable for portability, they suffer from the following disadvantages: